1. Field of the Invention
This invention generally relates to systems configured to provide illumination of a specimen during inspection. Certain embodiments relate to a system that is configured to provide illumination of a specimen during inspection that includes elements configured such that pupil distortions of the elements substantially cancel.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Fabricating semiconductor devices such as logic and memory devices typically includes processing a substrate such as a semiconductor wafer using a large number of semiconductor fabrication processes to form various features and multiple levels of the semiconductor devices. For example, lithography is a semiconductor fabrication process that involves transferring a pattern from a reticle to a resist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated in an arrangement on a single semiconductor wafer and then separated into individual semiconductor devices.
Inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield in the manufacturing process and thus higher profits. Inspection has always been an important part of fabricating semiconductor devices such as integrated circuits. However, as the dimensions of semiconductor devices decrease, inspection becomes even more important to the successful manufacture of acceptable semiconductor devices because smaller defects can cause the device to fail. For instance, as the dimensions of semiconductor devices decrease, detection of defects of decreasing size has become necessary since even relatively small defects may cause unwanted aberrations in the semiconductor devices.
One common way to improve the detection of relatively small defects is to increase the sensitivity of an optical inspection system. One way to improve the sensitivity of an optical inspection system is to use oblique illumination for wafer defect detection, instead of normal illumination. Several different oblique illumination configurations have been devised and used. Among them, laser line illumination using cylindrical lenses is one of the most cost-effective illumination configurations currently available because it can provide both a long illumination field that is necessary for high throughput and high spatial resolution in the direction perpendicular to the illumination line that is necessary for high sensitivity.
However, there are difficulties in achieving both high resolution and a long field due to pupil distortion. The pattern of pupil distortion looks like a smile or a frown pattern at the entrance pupil plane depending on its sign. Pupil distortion can be tolerated if the energy distribution at the pupil plane is uniform. However, in reality, all practical pupils have sharp boundaries, and the Gaussian profile of a laser beam makes the energy distribution even less uniform. If the energy distribution at the entrance pupil is not uniform, pupil distortion can cause variations in resolution and light intensity along the illumination line. Such variations adversely affect wafer defect detection.
Glass cylindrical lenses are typically used for illumination line formation. The total power of the cylindrical lenses in an illumination system must be positive to focus a laser beam into a line. However, positive power glass lenses introduce positive pupil distortion. In addition, strong positive power lenses that are needed and commonly used for fine illumination line formation generate a large amount of positive pupil distortion. To cancel the positive pupil distortion, strong negative power lenses must be used. However, strong negative power lenses create total internal reflections or a large amount of high order aberrations that degrade the performance of the system to an unacceptable level. For this reason, currently used line illuminators do not use high power negative elements and unavoidably have a large amount of pupil distortion. The pupil distortion severely limits the length of the usable segment of the illumination line and consequently reduces throughput and prevents efficient use of laser power.
Accordingly, it would be advantageous to develop a system configured to provide oblique line illumination of a specimen for inspection applications that has relatively low, or even no, pupil distortion such that substantially uniform resolution and light intensity along the illumination line can be obtained thereby providing high sensitivity and such that substantially an entirety of the illumination line can be used for inspection thereby providing high throughput and highly efficient use of a light source.